Polymerase chain reaction (PCR) is a technique discovered to synthesize multiple copies of one or more fragments of DNA from a single or multiple target templates. The original PCR process is based on heat stable DNA polymerase enzyme from Thermus aquaticus (Taq), which can synthesize a complimentary strand of a given DNA strand in a mixture containing the four types of DNA bases (cytosine, guanine, adenine, and thymine) and a pair of DNA primers, each primer at a terminus of the target DNA sequence. The mixture is heated to separate the double helix DNA into individual strands containing the target DNA sequence and then cooled to allow the primers to hybridize with their complimentary sequences on the separate strands and the Taq polymerase to extend the primers into new complimentary strands. Repeated heating and cooling cycles multiply the target DNA exponentially, for each newly formed double helix separates to become two templates for further synthesis.
A typical temperature profile for the polymerase chain reaction includes, (1) denaturation at 95° C. for 15 to 30 seconds; (2) hybridization with primers at appropriate annealing temperature for 30 to 60 seconds; and (3) elongation or extension of the hybridized primers at 72° C. for a period of time depending on the length of the DNA to be amplified, typically for about 30 to 60 seconds. The denaturation and hybridization steps occur almost instantly, however, in conventional PCR apparatus, the temperature changes at a rate approximately 1° C./second when a metal heating block or water is used for thermal equilibration. This conventional thermal cycle is inefficient for it requires heating and cooling of material other than the DNA sample itself.